linux-kernel - Re: [PATCH v12 07/17] iommu: Try to allocate blocking domain when probing device

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Message-ID: <Yw4Qzif8W53ykR6K@nvidia.com>
Date:   Tue, 30 Aug 2022 10:29:50 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Baolu Lu <baolu.lu@...ux.intel.com>
Cc:     Joerg Roedel <joro@...tes.org>,
        Christoph Hellwig <hch@...radead.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Ashok Raj <ashok.raj@...el.com>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Jean-Philippe Brucker <jean-philippe@...aro.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Vinod Koul <vkoul@...nel.org>,
        Eric Auger <eric.auger@...hat.com>,
        Liu Yi L <yi.l.liu@...el.com>,
        Jacob jun Pan <jacob.jun.pan@...el.com>,
        Zhangfei Gao <zhangfei.gao@...aro.org>,
        Zhu Tony <tony.zhu@...el.com>, iommu@...ts.linux.dev,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v12 07/17] iommu: Try to allocate blocking domain when
 probing device

On Tue, Aug 30, 2022 at 09:46:01AM +0800, Baolu Lu wrote:
> On 2022/8/30 01:27, Jason Gunthorpe wrote:
> > On Mon, Aug 29, 2022 at 11:40:24AM +0800, Baolu Lu wrote:
> > > On 2022/8/26 22:52, Jason Gunthorpe wrote:
> > > > On Fri, Aug 26, 2022 at 08:11:31PM +0800, Lu Baolu wrote:
> > > > > Allocate the blocking domain when probing devices if the driver supports
> > > > > blocking domain allocation. Otherwise, revert to the previous behavior,
> > > > > that is, use UNMANAGED domain instead when the blocking domain is needed.
> > > > > 
> > > > > Signed-off-by: Lu Baolu<baolu.lu@...ux.intel.com>
> > > > > Tested-by: Zhangfei Gao<zhangfei.gao@...aro.org>
> > > > > Tested-by: Tony Zhu<tony.zhu@...el.com>
> > > > > ---
> > > > >    drivers/iommu/iommu.c | 29 +++++++++++++++++------------
> > > > >    1 file changed, 17 insertions(+), 12 deletions(-)
> > > > This seems like a lot of overhead to allocate these things for every
> > > > group?
> > > > 
> > > > Why not add a simple refcount on the blocking domain instead and
> > > > allocate the domain on the pasid attach like we do for ownership?
> > > 
> > > I am working towards implementing static instance of blocking domain for
> > > each IOMMU driver, and then, there's no much overhead to allocate it in
> > > the probing device path.
> > 
> > Well, I thought about that and I don't think we can get
> > there in a short order.
> 
> Yes. Fair enough.
> 
> > Would rather you progress this series without
> > getting entangled in such a big adventure
> 
> Agreed. I will drop this patch and add below code in the iommu
> interface:
> 
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -3219,6 +3219,26 @@ int iommu_attach_device_pasid(struct iommu_domain
> *domain,
>                 return -ENODEV;
> 
>         mutex_lock(&group->mutex);
> +
> +       /*
> +        * The underlying IOMMU driver needs to support blocking domain
> +        * allocation and the callback to block DMA transactions with a
> +        * specific PASID.
> +        */
> +       if (!group->blocking_domain) {
> +               group->blocking_domain = __iommu_domain_alloc(dev->bus,
> +                               IOMMU_DOMAIN_BLOCKED);
> +               if (!group->blocking_domain) {
> +                       ret = -ENODEV;
> +                       goto out_unlock;
> +               }
> +       }
> +
> +       if (!group->blocking_domain->ops->set_dev_pasid) {
> +               ret = -EOPNOTSUPP;
> +               goto out_unlock;
> +       }
> +
>         curr = xa_cmpxchg(&group->pasid_array, pasid, NULL, domain,
> GFP_KERNEL);
>         if (curr) {
>                 ret = xa_err(curr) ? : -EBUSY;
> 
> Currently both ARM SMMUv3 and VT-d drivers use static blocking domain.
> Hence I didn't use a refcount for blocking domain release here.

I don't think that works in the general case, you can't just destroy
what is in group->blocking_domain..

Maybe all of this is just the good reason to go to a simple
device->ops->remove_dev_pasid() callback and forget about blocking
domain here.

Jason